The present invention relates generally to gas seals which surround rotating shafts and, more particularly, to gland seals which utilize a thin film of oil which is maintained between the rotating shaft and two floating rings.
During the past 60 years, the development of hydrogen-cooled dynamoelectric machines has advanced significantly. Since the issuance of U.S. Pat. No. 1,453,083 to Schuler on Apr. 24, 1923, progress in hydrogen cooling of electrical generators has occurred steadily, along with suitable sealing devices which prevent the escape of hydrogen gas along the rotating shafts of the machine at the locations where the shaft extends axially from the gas-tight stator housing. The sealing of this interface between the stationary and rotating components of a dynamoelectric machine is generally accomplished by the use of a gland seal.
A hydrogen-cooled dynamoelectric machine requires shaft gland seals at each end of the generator's rotor in order that the hydrogen gas be prevented from escaping from the generator housing and also in order that air is not permitted to enter the generator housing. Gland seal assemblies generally comprise a bracket member and one or more sealing rings which are fitted close to the shaft's circumference and are free to move with the shaft in a radial direction. Generally, these rings are also permitted a slight axial movement along the shaft surface, but are retained so that they cannot rotate around the shaft. Sealing oil is provided at the interface between the shaft and rings in order to prevent the disadvantageous passage of gas through this gap. In typical gland seal designs, oil is pumped in a radially inward direction through the ring assembly and toward the rotor surface. After contacting the rotating surface of the rotor, the oil then travels in opposing axial directions and produces a thin oil film between the shaft and the gland seal rings.
The oil must be provided at a pressure which exceeds both the hydrogen pressure within the generator and the atmospheric pressure of the air at the axially outboard region of the gland seal assembly. The oil leaving both the air and hydrogen sides of the gland seal assembly is then collected and returned to a gland seal oil reservoir. It is generally necessary to remove gas and moisture from the gland seal oil supply by a vacuum treating process in order to prevent the oil which is leaving the hydrogen side of the shaft gland seals from liberating this gas and moisture inside the generator housing and thus reducing its hydrogen purity. Gland seal oil systems have been described at great length in the technical literature. For example, the early development of gland seals is discussed in "Hydrogen Cooled Turbine Generators" by M. D. Ross and C. C. Sterrett, volume 59, AIEE Transactions, Jan., 1940, pps. 11-17. Another discussion of the sealing of hydrogen cooled generators can be found in "The Hydrogen Cooled Turbine Generator" by D. S. Snell, Volume 59, AIEE Transactions, Jan. 1940, pps. 35-50. The dynamics of oil seal systems has been analyzed in "Liquid Film Seal for Hydrogen Cooled Machines" by C. W. Rice, General Electric Review, Volume 30, No. 11, Nov. 1927, pps. 516-530. An oil purification system used in conjunction with gland seals is described in "Continuous Scavenging System for Hydrogen Cooled Generators" by D. S. Snell and L. P. Grobel, AIEE Transactions, volume 69, 1950, pps. 1625-1636.
As discussed in the above cited references, hydrogen is typically sealed in generators at each end of its rotating shaft by means of a continuous film of oil maintained between the shaft and one or more floating rings. Two rings are generally used on smaller generators with lower gas pressures, and oil is introduced between the two rings at a pressure which is higher than the contained hydrogen gas pressure. The oil then passes between the shaft and the seal rings in both axial directions. The oil which flows towards the generator is then contained in a defoaming tank and eventually drained back to the sealed oil system where a vacuum treating system removes entrained hydrogen which has been absorbed by that oil. When high hydrogen pressures are used, the loss of hydrogen can be excessive and expensive. This hydrogen loss is a function of the quantity of oil flowing toward the hydrogen side of the seal. In order to minimize this hydrogen loss on larger generators, double oil flow systems were developed in the 1950's. Double flow oil systems use two separate oil flows. One oil flow is directed toward the air side of the seal and the other is directed toward the hydrogen, or generator, side with a small buffer zone in between. These two oil systems are designed so that their pressures can be generally equalized, thus minimizing both the introduction of entrained air into the hydrogen and the loss of hydrogen by entrainment in the oil. A detailed description of simple flow and double flow gland seal systems is contained in "Gland Seal Systems for Modern Hydrogen Cooled Turbine Generators", By R. A. Baudry and L. T. Curtis, which was presented at the AIEE Winter General Meeting, N.Y., Jan. 21-25, 1957, pps. 1-10.
A double flow system which is presently used on generators with gas pressures of 45 psig or more utilizes a single ring which has two oil feed grooves, one for air side oil and the other for hydrogen side oil. This ring is contained in an annular groove of a gland seal bracket and is free to move with the shaft in a radial direction by virtue of an axial clearance between the ring and the annular groove. This freedom of movement is important because a ring which is bound within the bracket can cause rubbing between itself and the shaft and, thus, induce unstable shaft vibrations. In order to maintain this freedom of movement, not only does the ring require a slight axial clearance within the annular groove, but the frictional forces between the ring and the groove must be minimized. This frictional force is partly a function of the axial pressures acting on the ring and pushing it against the air, or outboard, side of the annular grooove. Normally, air side oil is also introduced into a float oil groove and the net unbalanced axial force is thus minimized to a sufficient degree.
Significant problems are experienced in conjunction with presently known gland seal designs. Excessive oil can be introduced into the generator either by intermittent oil spills or on a regular continuing basis. The normally expected oil flow toward the hydrogen side of the seal ring at any given differential seal oil pressure has been found to be of prime importance in the generator's oil usage. This oil flow creates vapor in proportion to its quantity and must be drained in order to avoid spills within the generator housing. Therefore, it is significantly advantageous to reduce this oil flow.
In present designs, it has sometimes been found that more than half of the flow of oil to the hydrogen side passes in a radially inward direction between an axial face of the gland seal ring and its proximate axial surface of the annular groove of the gland seal bracket. The clearance between the gland seal ring and the bracket is generally held to a nominal 0.007 inches to permit the radial movement of the gland seal ring as described above. This escaping oil, which does not beneficially aid the primary function of the gland seal system, travels a radial distance of approximately 0.37 inches from a hydrogen side oil feed groove to the hydrogen gas atmosphere within the generator frame. The significance of this oil loss can be realized by comparing this oil path to the axial clearance between the ring and the shaft which is only about 0.0025 to 0.0035 inches over a length which is approximately 0.625 inches long in the axial direction. Since any oil flow through this type of clearance is proportional to the cube of the clearance and inversely proportional to the length of passage and the oil viscosity, it can be calculated that this non-functional radial flow exceeds the functional axial flow and can constitute over half the total flow into the generator under these conditions.
Another significant problem encountered is shaft vibrations which are caused by a rubbing between the shaft and the gland seal rings. Occasionally these vibrations require disassembly of the bearing and gland seal brackets in order to determine their precise cause. One possible cause of this rubbing is the fact that the ring is not free to move within its annular groove due to the loss of the axial clearance described above. This axial clearance can be lost due to either bracket or ring distortion, the ring being out-of-round or the ring being assembled in a non-perpendicular association with the shaft. The problem of rubbing between the ring and the shaft is related to the above-described problem of oil usage in that, if a larger axial clearance is used between the ring and its bracket to prevent binding the ring, even more oil will flow into the generator.
Since gland seal rings are typically made from two ring halves which are butt-jointed and bolted together, variations in bolt tension can produce significant variations in ring roundness. Since, in single ring gland seal systems the ring is approximately 1.75 inches wide in the axial direction, it is very important that the ring be perpendicular to the shaft center line in order to minimize possible susceptibility to rubbing. When the gland seal ring is not perpendicular to the shaft center line, there is not only a greater possibility of rubbing between the gland seal ring and the shaft, but also a pumping action in the oil film that can increase oil flow into the generator.
It is an object of the present invention to provide a gland seal assembly which permits adequate radial movement of the gland seal ring while minimizing excessive oil usage which is caused by the passage of gland seal oil in a radially inward direction between the bracket and ring. Another object of the present invention is to permit the use of air side oil to cool the gland seal rings. A further object of the present invention is to minimize the importance of close machining tolerances of the annular groove of the gland seal bracket and to permit the gland seal rings to be assembled without significant rish of distortion. The present invention provides a gland seal assembly which utilizes a double oil flow and two gland seal rings. Each of the gland seal rings has an inner cylindrical surface and the two rings are assembled with their inner cylindrical surfaces being coaxial. Both rings are disposed in an annular groove of a gland seal bracket. The gland seal assembly of the present invention provides an oil flow between the first and second rings in a radially inward direction toward the shaft surface. The second gland seal ring is provided with a conduit which allows fluid communication between a passage through the bracket and the oil film which is located between the shaft and the second gland seal ring.
The inner cylindrical surface of the second gland seal ring is provided with an opening through it. This opening is in fluid communication with the conduit which permits oil passage through the second ring. Therefore, a flow of oil is permitted to pass from an external source through the gland seal bracket, through the second gland seal ring and into the clearance gap which exists between the second gland seal ring and the rotor. This oil flow is maintained separately from another oil flow which travels from an external source, into the annular groove of the bracket, between the first and second gland seal rings and into the clearance gap which exists between the first gland seal ring and the shaft. The first oil flow travels axially inward along the rotor shaft and towards the hydrogen atmosphere within the generator frame. The second oil flow travels at an axially outward direction along the rotor shaft and toward the air at atmospheric pressure. The passage of the second oil flow between the first and second gland seal rings causes the two rings to move away from each other in opposing axial directions. This movement causes each of the rings to contact opposing axial sides of the annular groove of the gland seal bracket and thus minimize oil passage between each of the gland seal rings and its most proximate axial surface of this annular groove.
The first gland seal ring can also be provided with an additional groove on its axial surface which is most proximate an axially outward surface of the annular groove of the gland seal bracket. The function of this groove is to permit the air side oil of the second oil flow to exert a slight axial force between the gland seal bracket and the first ring. This force is exerted against the first gland seal ring in an axially inward direction and acts to counterbalance other axial forces in the opposite direction. The purpose of this counterbalancing is to prevent binding between the first gland seal ring and the outward axial surface of the annular groove of the bracket.
The present invention also provides for the second oil flow to pass over the radially outward surfaces of the second gland seal ring. The purpose of this passage is to provide additional cooling of the second gland seal ring. The second oil flow passes over these surfaces of the second gland seal ring prior to its passage between the first and second gland seal rings in a radially inward direction towards the shaft.
Since the first and second oil flows are kept at essentially identical pressures, the rate of the first oil flow will be significantly less than the second because the hydrogen gas within the generator frame is at a significantly higher pressure than the air towards which the second oil flow passes. Therefore, the second flow of oil experiences a significantly higher pressure differential which causes a higher rate of flow. The cooling capability of this higher rate of flow of the second oil flow is taken advantage of by passing it over the second gland seal ring prior to its entry into the clearance gap between the first gland seal ring and the shaft.
Since gland seal rings are usually manufactured in two halves and bolted together during assembly, the present invention incorporates a lap-type joint between these two halves in contrast with the butt-type joint used in present single ring designs. The importance of this joining technique is that, during assembly of the two halves, the former method exerts a tangential force on the ring halves. If the joined surfaces of these two ring halves are not perfectly aligned, this tangential force will tend to distort the ring in order to align these mating surfaces. In the present invention, the lap-type joint requires only axial forces and, therefore, will not distort the gland seal ring even if the mating surfaces of its two halves are misaligned.